Prestressing wire and method of manufacturing the same



July 20, 1965 K. G. HANN 3,196,052

PRESTRESSING WIRE AND METHOD OF MANUFACTURING THE SAME Filed June 28,1961 5 Sheets-Sheet 1 5 m0 WIRE AFTER PRocEssme 6 E g \wms BEFOREPRocEssms. a

E 40 I G- 3 PERCENTAGE OF ELONGAT\DN.

July 20, 1965 K. G. HANN 3,196,052

. PRESTRESSING WIRE AND METHOD OF MANUFACTURING THE SAME Filed June 28,1961 5 Sheets-Sheet 2 Jig. Z);

July 20, 1965 NN 3,196,052

K. G. HA PRESTRESSING WIRE AND METHOD OF MANUFACTURING THE SAME FiledJune 28, 1961 5 Sheets-Sheet 5 Z a O 0 DQocEssED AT 290 C. QELAXED AT 20G.

g mmAL LOADING norms/5 \b 9 5 0 PROCESSED AT 200 C. 03 \QELAXED AT 20C mINITIAL LQADING lOO TON/5QJN,

*- 0 2000 4000 woo 8000.

DUQATION United States Patent Office 3,l%,052 Patented July 20, 19653,196,952 PRESTRESSEIG WEE AND METHGD OF MANUFACTURING THE SAME KennethGraeme Hann, Hendrescythan, Creigiau, near Cardifi, 'Glarnorgan, Wales,assignor to Somerset Wire Iompany Limited, Bridgwater, England, aBritish company Filed lune 28, 1961, Ser. No. 12%,441 Claims priority,application Great Britain, lune 1, 1%3, 15,235/53; Aug. 10, 1953,22,055/53 8 Claims. (Cl. 143-12) This application is acontinuation-in-part or" my application Serial No. 43 1,965, filed May24, 1954, now abandoned, for Prestressing Wire and Method ofManufacturing the Same.

This invention relates to a new or improved method of straighteningmetal wire.

The present invention is concerned with the straightening of cold drawnhigh tensile steel wire which is formed of plain medium carbon and highcarbon steel as opposed both to alloy steel, and also to mild steel, thetensile strength of which latter steel is too low for it to be usefulfor the purposes herein described.

The invention is concerned in particular with medium and high carbonsteel wire having a carbon content within the range of 0.35% to 0.9%. Insuch medium and high carbon steels, manganese and silicon are present byreason of having been added as deoxidising agents in accordance withordinary steel manufacturing practice, the manganese content usually notexceeding 0.7% and rarely, if ever, exceeding 1.0% and the siliconcontent usually not exceeding 0.4% and rarely, if ever, exceeding 0.8%,the sulphur and phosphorous contents of which usually do not exceed0.04% and 0.03% respectively, and the maximum permissible upper limit ofthese two latter elements being in general 0.06% in each case; with ironand the usual residual, i.e., commercial impurities forming the balanceof the composition. Medium and high carbon steel having a composition asdescribed in this paragraph, in particular a carbon content within theabove range of 0.35% to 0.9% is herein referred to as plain carbonsteel.

The subjection of plain carbon steel wire to a straightening operationis advantageous for a variety of reasons, namely:

(i) Ease of manipulation, i.e., no allowance need be made for varyingdegrees of curvature in the subsequent manipulation operations to whichthe wire may be subjected.

(ii) In certain particular applications it is very desirable that thewire should be as straight as possible. One specific case is that ofwire spokes for wheels.

A further specific and very important application of the invention is inthe production of pre-stressing wire for reinforced concreteconstruction wherein it is of especial importance that the wire shouldnot only be of a markedly straight configuration but should also possessparticularly good tensile properties including a high ultimate tensilestress and the maximum resistance to creep under a continued tensileloading. That is to say in such application to pre-stressing wire, it ismost important that the wire should possess a low stress relaxation,that is to say, the minimum of reduction or falling off of its tensilestress from some initial maximum, during the continued tensile loadingof a piece of wire, the length of which is kept constant. In otherwords, the strain in the wire therein is maintained at a constant valueof progressively reducing the tensile loading, i.e., the tensile stressin the wire and over an extended period of time.

As a pro-stressing wire is required to maintain a predetermined tensileloading on the concrete, such stress relaxation characteristic is ameasure of the, usefulness of the wire as a pro-stressing wire.

(iii) In certain specific operations in which wire is fabricated thestraighter the wire the simpler and easier the fabrication operation,for example, in the coiling of wire in the production of coiled springs.

It is already known to straighten cold drawn plain carbon steel wire byadvancing the wire in the cold state, i.e., at room temperature, throughoifset dies in a rotary straightening machine. Such a method in whichthe wire is straightened cold results in significant impairment ofcertain of the mechanical properties of the wire. In particular there isa marked lowering of the proof stress and some lowering of the ultimatetensile strength of the wire.

For instance the following test was carried out by myself on plain highcarbon steel wire having the following composition:

Percent Carbon 0.8 Manganese 0.7 Silicon 0.2 Sulphur and phosphorus 0.05

iron and the usual commercial impurities the re mainder.

1 Maximum in each case.

This wire of diameter 0.2" was advanced in the cold state through arotary straightening machine as aforesaid, and I found a reduction inthe 0.1% proof stress of between 10 and 15% and a reduction in theultimate strength of the wire by substantially 5%.

A principal object of the present invention is the provision of a new orimproved method of straightening cold drawn plain carbon steel wire,which method avoids the aforesaid impairment of certain of themechanical properties of the wire as occurs if the wire is straightenedin the cold state by the existing technique above referred to, whichmethod in accordance with this invention serves incidentally to improvesubstantially certain of the mechanical properties of the wire, namelyin particular to improve substantially its creep resisting properties under tension as well as to increase the elongation of the wire beforefailure under tension.

A further object of the present invention is to provide an improvedmethod of manufacture of a plain carbon steel pro-stressing wire bywhich the wire is characterized by a particularly low stress relaxation.

A further object of the present invention is to provide a new orimproved method of manufacturing a plain carbon steel pro-stressing wirewhich is characterized by having a particularly high ultimate tensilestrength.

Other objects of the present invention will become apparent from aconsideration of the specification, including its accompanying drawingsand the appended claims.

The present invention in its essentials comprises a method ofstraightening a cold drawn plain carbon steel wire and having a carboncontent within the aforementioned range of 0.35% to 0.9%, and preferablywithin the range of 0.4% to 0.8%, wherein the wire after the colddrawing operation is heated to a tempering temperature within the rangeof 150 C. to 500 C., and preferably within the range of 200 C. to 400C., and a controlled tension is applied to successive lengths of the soheated wire, such tension being of magnitude sufiicient to effect apermanent elongation and straightening of the wire without subjectingthe wire to such a high tensile stress as to cause it to fracture.

The foregoing method, in addition to producing a straightening effect onthe wire, also results in relieving local stresses by the subjection ofthe wire to a tempering operation, and improves significantly thetensile properties of the wire, notably by increasing its ultimatetensile stress and improving its creep resistance and stress relaxationunder continued tensile loading.

In order that the meaning of certain expressions em ployed in thisspecification may be clearly understood, these are defined in thefollowing:

DEFINITIONS H (1) By the expression permanent elongation employed hereinis meant that the elongation imparted to the wire when heated to atempering temperature as defined below is at least partially retainedafter the wire has cooled to normal, i.e., room temperature.

In other words the elongation of a given length of wire which isproduced is greater than that expected from the sum of the strain in thewire due to the applied tensile load and the linear expansion due to therise in temperature.

(2) By the expression tempering temperature is meant a temperingtemperature as customarily understood in the art of heat treating theplain carbon steel to which the invention is applied, that is to say itis a temperature within the tempering range to which the steel would beheated after a quenching operation for the purpose of increasing itsductility.

Such temperature will necessarily, of course, be below the lowesttransformation point in the iron carbon diagram and will necessarilyvary in accordance with the analysis of the particular steel concerned.In the case of medium or high carbonsteel having a carbon content withinthe range of 0.35% to 0.9% the tempering temperature would, as is wellunderstood, be within the range of 150 C. to 500 C., and as hereinafterexplained would preferably be within the range of 200 C. to 400 C.

(3) By the expression straightening as used herein is meant that theshape of the wire which is subjected to the straightening operation ismade to conform more closely to an absolutely straight configuration,but such expression is not herein intended necessarily to imply that'thewire is so shaped by the method according to this invention as to beperfectly straight, although it is believed that in fact by the methodaccording to this invention it is possible to produce wire. in a formwhich closely approximates to that of absolute straightness.

(4) 0.1 proof stress.-The tensile stress required to produce a permanentelongation of 0.1% in the wire. Such proof stress is commonly measuredduring an ordinary tensile test as customarily practised.

The primary advantages of the present invention may be summarised asfollows:

(a) It provides a method of straightening wire formed of cold drawnplain carbon steel in which the wire can be straightened to a formcomparable with that obtainable by a cold straightening method using arotary straightening machine as above described, wherein the impairmentof certain of the mechanical properties of the wire attendant on suchhitherto known method as above indicated are avoided.

(b) The wire is much less liable to creep, i.e., much 4 more stableunder conditions of long period or continuous tensile loading.

(0) The elongation of the wire before failure is increased as comparedwith an otherwise similar wire produced by normal drawing processes.

(d) The elastic limit or proof stress of the wire can be markedlyincreased in comparison with that of an otherwise identical wire butproduced by normal drawing processes.

(e) The impact resisting properties of the wire are improved.

Although the invention is capable of a general application of which onevery important application is in the production of wires for use in themanufacture of wire spokes the invention is especially applicable to theproduction of wires for use in pre-stressed concrete as not only doesthe particularly straight configuration of the wire facilitate its usefor this purpose, but as hereinbefore explained under paragraphs (b) to(d) inclusive above, the improvement in the tensile properties make itespecially suitable for this work.

At present it is customary in pre-stressed concrete to employ as thepre-stressing members, wire formed of plain carbon steel as hereindefined, and produced by ordinary cold drawing operations, and theseentail the disadvantage that when they are subjected to a tensile loadequal to half or more than half of the breaking load, the elongation ofthe wire increases with time and concurrently the stress in the wireheld at a constant strain relaxes with time. These are phenomena whichare particularly undesirable in pre-stressed concrete construction,which disadvantageous phenomena are avoided, or substantially avoided,by the present invention.

As above explained the present invention is essentially concerned with amethod of straightening cold drawn wire formed of plain carbon steel,and although the method may be applied to such wire which has previouslybeen subjected to a cold drawing operation, the cold drawing operationand the straightening method in accordance with the present inventionare preferably combined, so that the drawing down die or the reducingrolls employed in the cold drawing operation may co-act with the wirepulling means employed in producing the required tension, such pullingmeans comprising various known forms of gripping devices, for examplecaterpillar gripping jaws, or a power driven rotating block or capstan,around which the wire would be wound continuously after itsstraightening.

Thus the apparatus required for carrying out the method the subject ofthis invention would then constitute an auxiliary part of the drawingdown plant.

The preferred method of heating the wire or the like is by directresistance heating, i.e., by passing an electric current through thatpart of the wire or the like which is being subjected to the tensileloading, but other methods of heating may be used such as inductionheating, or by passing the Wire through the lead orsalt bath, or throughsome form of furnace.

The invention may be applied to wire or the like of plain configuration,or it may be applied to wire which is of indented configurationperipherally, and the indenting operation may be combined with thestraightenin method the subject of this invention. a

As above set forth the invention is applicable to plain, medium and highcarbon steels as distinct from alloy steels and comprising a carboncontent within the range of 0.35% to.0.-9%. The minimum'valuepf thisrange is determined by the fact that with a carbon content below 0.35%the ultimate tensile strength of the steel is too low for the steel tobe commercially useful in the production of wire required to possessmarked tensile strength, as in the case, for example, of pre-stressingwire, and for the foregoingreason 0.4% carbon represents the minimumuseful carbon content for many applications of plain carbon steel wire.The upper limit of 0.9% for the carbon range is determined by the factthat 0.8% carbon is the eutectoid composition for iron carbon, andcarbon in amount greater than 0.8% tends during the production of thewire to form at the grain boundaries pre-eutectoid cementite whichreduces the ductility of the metal and makes the drawing operation moredifficult. For this reason 0.8% carbon is the preferred upper limit forthe carbon range.

In order that the invention may be more fully understood, reference isdirected to the accompanying drawings, wherein:

FIGURE 1 shows a schematic layout of one form of plant for carrying outone specific method in accordance with the present invention;

FIGURES 2, 3 and 4 illustrate modifications of the plant depicted inFIGURE 1 for carrying out respectively three modified forms of the saidmethod;

FIGURE 5 shows a graph illustrating the improvement in certain physicalproperties obtained in the finished wire when subjected to thestraightening method forming the subject of this invention;

FIGURES 6, 7, 8 and 9 are graphs depicting the relaxationcharacteristics of pro-stressing wires of a number of differentanalyses, and which wires have been subjected to the method of thisinvention.

Referring firstly to FIGURE 1 of the drawings, the invention is heredepicted as applied to the straightening of plain carbon steel wire soas also to improve the tensile properties thereof and to render the wireparticularly useful for pre-stressed concrete construction. The wire isfirst advanced in the cold state through one or more drawing down orreducing dies, one only of which dies of conventional form is indicatedat 10 in the drawing, such die or dies serving to effect reduction inthe cross section of the wire and thereby effectively to grip it. Thewire is advanced continuously through this die 10 by means of acontinuously rotating power-driven capstan or block 11, around which thewire is coiled by the continued rotation of the capstan or blocked inthe known manner. This capstan or block 11 is generally of conventionalform but has a diameter which is substantially larger than that normallyused, a particularly convenient diameter to be employed being 350 to 500times the wire diameter, i.e., a diameter of 100" in the case of wire of0.2 diameter, which 100" diameter is substantially four times largerthan that normally used.

The length of the wire advancing continuously from the drawing die tothe capstan or block is depicted at 12.

The drawing die 10 is connected through lead 13 to one side of a motorgenerator of known form indicated diagrammatically at 12 and adapted tosupply alternating or direct current at a low voltage, conveniently 20to 40 volts, and at a substantial amperage, for example, of the order of200 to 500 amperes.

The other side of this motor generator is connected by lead 15 to aplurality of spring loaded current collecting brushes 16 which are inelectric conducting engagement with the wire 12 at a positionintermediate the drawing die 10 and the capstan or block 11, thesebrushes being so arranged as to permit of the wire advancing freely inrelation thereto as it is coiled around the capstan or block.

The arrangement is such that a heavy current of the value aboveindicated is thereby passed along that part of the wire 12 which isbetween the brushes 16 and the drawing die 10.

The side of the motor generator 14 which is connected to the drawing die13 is further connected by another lead 17 through adjustable resistance10 and brush gear 19 to the rotating capstan or block 11 so as therebyto pass current along that part of the wire 12 which is between thecapstan or block 11 and the brushes 16. The adjustable resistance 18 isadjusted so as to maintain the wire 12 as it advances from the brushes16 towards the capstan or block 11 at a substantially constanttemperature, and a convenient current to pass along the wire between thebrushes 16 and the capstan or block 11 for this purpose is one of theorder of 50 to amperes, i.e., of magnitude substantially less than thatpassed along that part of the wire which is between the drawing die 10and the brushes 16.

The arrangement is in fact such that a heavier current is passed alongthat part of the wire which is advancing from the drawing die to thebrushes so as to raise this part of the wire relatively rapidly to thedesired temperature, and the wire is subsequently maintained at suchtemperature for a predetermined period of time by the passage of thelesser current therealong between the brushes 16 and the capstan orblock 11.

The maximum temperature to which the wire is heated is in fact atempering temperature, namely, a temperature within the range of to 500C., the preferred value of this temperature within the foregoing rangebeing dependent upon a number of different factors as follows:

(a) The composition, in particular the carbon content of the plaincarbon steel employed. The lower the carbon content, the lower thetensile stress necessary to effect the desired permanent elongation andstraightening of the wire, particularly at the lower temperatures withinthe above range of 150 C. to 500 C.

(b) The desired tensile properties of the wire. In general theapplication of a higher tensile stress with a heating temperaturetowards the lower end of the temperature range produces a particularlyhigh ultimate tensile stress in the wire.

(c) The tension which is applied to the wire by means of the rotatingcapstan or block. The higher the temperature, the lower the tensilestress necessary to produce a given permanent elongation in the wire.Conversely, the greater the tension the lower the temperature which cansatisfactorily be employed, it being understood that if the tension istoo great then failure of the wire under tension will occur.

Set out below is the result of some experimental measurements of thetensile stress necessary to produce a permanent elongation of 1% andalso 2% in plain carbon steel wire heated to varying temperatures withinthe temperature range above mentioned:

1% elongation 2% elongation The analysis of the wire employed in theforegoing tests was as follows:

Percent Carbon 0.76 Silicon 0.23 Manganese 0.63 Sulphur 0.02 Phosphorus0.02

Iron and residual impurities the remainder.

(d) The desired stretch, i.e., permanent elongation of the wire. Theminimum usefulpermanent elongation is 1%, and the maximum permanentelongation which I find to be advantageous is in general about 5%.

As applied to wire formed of plain carbon steel, I find thatsatisfactory results are obtained if the wire is heated to a maximumtemperature which is within the range of 150 to 500 C., and preferablywithin the range of between 200 to 220 C. as the lower limit, and 400 C.as the upper limit, one especially preferred temperature range where thecarbon content is approximately 0.8% being 350 to 400 C.

The definedlimits of 150 C. and 500 C; are determined as follows. Thelower temperature limit is determined by that fact that:

(i) Below this limit the maximum ultimate tensile strength of thestraightened wire commences to fall, the ultimate tensile strength beingat a maximum in the case of a carbon content of about 0.8% where theheating temperature is around 150 C. to 200 C.

(ii) 150 C. represents the lowest practical temperature limit at anyrate with the higher carbon contents, e.g., 0.7 to 0.9% at which thewire under commercial conditions of operation can be stretched withoutfracture, to produce a useful permanent elongation, i.e-., of at least1%.

The defined limit of 500 C. represents the maximum temperature at whichthe stretching of the wire can be eifected consistent with the retentionof a sufficiently high ultimate tensile strength for the process to beuseful.

- In Table D which appears later herein there 'is set out certain testresults at various temperatures within the foregoing range, and whichsubstantiate the selection of the foregoing maximum temperature limitsof 150 C., to 500 C., as well as the foregoing preferred lower limit of200 to 220 C. below which temperatures the ultimate tensile stresscommences to decrease, while quite a marked decrease in ultimate tensilestress occurs where the temperature at which the wire is stretched isabove 400 C.

In order to prevent the wire heated to the foregoing temperature onsubsequent cooling contracting on to the block or capstan prior to itsremoval therefrom, so as seriously to impair the removal of the wire, Iprovide means for cooling the wire immediately prior to its advancementaround the capstan or block.

Such means comprise a tube 20 through the interior of which the wirepasses, the tube having a bore substantially greater than the diameterof the wire and the end of the tube nearest to the capstan or blockbeing partially closed 'so as to provide therein merely a central hole21 of bore slightly greater thanthe diameter of the wire so as toprovide a guide for the wire and maintain it substantially centralwithin the tube. Adjacent this end of the tube 20 this is provided withan inlet 22 for cold Water which is fed continuously through the tube,the water flowing out at the opposite end 23 thereof, and therebyserving to cool the wire down to a temperature at which it can be coiledwithout fear of'it gripping the capstan or block unduly tightly.

The tensile load is applied to the wire by the action of the powerdriven capstan or block 11 in advancing the wire through the drawing dieand the reduction in cross section at the drawing die must besufliciently great as to enable this to exert the necessary grip on thewire for this purpose, so that the capstan or block may exert on thewirea tensile force sufficiently great as to produce the elongation inthe length of the wire heated to the temperature aforesaidwhich isgreater than the sum of the strain in the wire due merely to the tensileload and the linear expansion due to the rise in temperature. Otherwisethe desired straightening of the wire would not be effected, andconcurrently the above referred to improved creep resisting andelongation characteristics of the wire under tension would not beobtained. v

In the case of a plain carbon steel wire having a carbon content of0.76% which is heated to a temperature within the particularly preferredlimits of 300 C. to 400 C., the

tensile loading which is applied to the wire may be within the range ofapproximately 35% to 70% of the breaking load, and preferably is between35% to of the breaking load, i.e., with a carbon steel .wire having acarbon content of 0.76% and a breaking load of approximately 100 tonsper square inch the tensile loading applied may correspond to' a tensilestress in the wire of approximately 50 tons per square inch. a

1 Although as above described it is preferred to carry out the method ofthe subject of this invention as a continuation of the cold drawing downprocess, in that the reducing die or. rolls employed provide thenecessary anchorages for the one end of the wire which is beingprocessed, the invention as shown in FIGURE 2 may be similarly appliedto wire which has previously been drawn down by taking a length of suchdrawn down wire, winding it around a feeding drum 24 provided with abrake or other means for retarding its rotation, the wire being fed fromthis feeding drum 24 to thepower driven capstan or block 11 andsubjected in so doing to heating, tension, and cooling by means arrangedin exactly the same way as in the case of the plant depicted in FIG- URE1, as will be apparent from this FIGURE 2 of. the drawing; I In applyingthe invention to galvanised carbon steel wire, the brushes 16 by reasonof the slidable engagement' of the heated wire therewith would impairthe galvanised coating, and to avoid this in the case of galvanisedwire, the arrangement depicted in FIGURE 3 is adopted in which thecarbon brushes I6 and lead 21 are omitted entirely and the lead 15 isconnected direct to the brush gear 19, that is to say the heavy currentof, for example, 300 to 500 a'rnperes fiows from the whole of the wirewhich is between the drawing die 10 and the capstan or block 111.

The arrangement shown in FIGURE 3 may advantageously be applied to wirewhich is not galvanised in that this arrangement is a simpler one thanthat of FIGURE 1 by its omission of the brushes 16.

The invention may also be applied to indented wire which, after theconclusion of the drawing down operation, is indented in the knownmanner by passing it, as shown in FIGURE 4, between a pair of indentingrolls 25 of known form provided in the usual way with a series ofperipheral indentations or recesses. Such indented wire is advantageousas applied to pre-stressed concrete.

In this latter arrangement depicted in FIGURE 4, the

I drawing down die 10 is replaced by one or more sets of reducing rollsindicated diagrammatically at 26, which serves to elIect equal reductionof the wire, and these reducing rolls may replace the drawing down die10 with the arrangements depicted in FIGURES 1 and 3.

In this lastarrangement depicted in FIGURE 4, one lead 13 from the motorgenerator set 14 is taken to the indenting rolls 25, otherwisethearrangement is identical with that depicted in FIGURE 3. I 7

With both arrangements depicted in FIGURES 3 and 4 in which the fullcurrent flows along the entire length of the wire, including that partthereof which is between the cooling tube and the capstan or block, it iimportant that the'cooling tube 17 should be disposed as close to thecapstan or block 11 as is practicable so as to reduce the length of thecooled wire between the tube 17 and the capstan or block, through whichthis full current is flowing, and thereby to avoid undesirable reheatingthereof.

With any of the above arrangements, if the reduction of area of crosssection produced by the drawing die 10 or drawing down rolls '26 isunduly low it may be necessary to augment the. load reaction thereby puton the encased wire in various known Ways, namely, by passing the drawndown Wire through rolls to which is attached a brake, or by passing thewire through rolls, which cause reverse bending, or by using a pluralityof drawing down dies or sets of drawing down rolls arranged in serieswith one another. The object, of course, in using any of such devices issimply to increase the tension in the wire sufiiciently to obtain wirewhich is as straight as possible.

Further, in place of the motor generator set 14 there may be used atransformer for supplying low voltage alternating current at the desiredcurrent density.

Three specific examples of pro-stressing wire formed of cold drawn plaincarbon steel and processed, i.e., subjected to the method of thisinvention, using the particular plant depicted diagrammatically inFIGURE 3 of the accompanying drawings, are as follows:

Table A Analysis, percent Examplel l Example H i Example III 0. 70 O. 400. 51 0. 23 0. 061 0. 203 0. 021 0. 037 0. D45 0. D24 0. 028 O. 029Manganese. 0. 63 0. 69 0. 61 Iron and re dual impurities t) 1 Theremainder.

.2 3 6 inch.

.202 inch.

Comparative tensile tests were performed on wire in accordance with eachof the three examples, these comparative tests being performed in thecase of each of the three examples on wire subjected to the method thesubject of this invention, under the conditions specified in Table A,and hereinafter referred to as processed wire, together with otherwiseidentical wire, i.e., of the same analysis manufactured under the sameconditions and cold drawn to the same final diameter of 0.202"; suchlatter wire in each case being not, however, subjected to heating undertension in accordance with the present invention. Such latter wire forconvenience in description is herein referred to as unprocessed wire.

The results of these tensile tests are set out in the following table B.

The improvement in the 0.1% proof stress, in the ultimate tensilestress, and in the percentage elongation of 10 the processed wire ineach of the three examples as compared with the otherwise identicalunprocessed wire is significant in every case.

In the case of Example I, the improvement of the tensile propertiesobtained with the processed wire as compared with the unprocessed wire,is further illustrated by the graph depicted in FIGURE 5 of theaccompanying drawings, wherein is depicted two stress-strain curves forwire of the foregoing specific composition cold drawn down in each caseby the final drawing down die to a diameter of .202, wherein the dashedline depicts the stressstrain curve obtained with the wire after drawingdown before processing, and the continuous line depicts thestress-strain curve of this same wire after both drawing down andprocessing in accordance with the foregoing specific example.

Wire in accordance with each of the three foregoing examples, wassubjected to a series of stress relaxation tests which were performed atboth room and elevated temperatures, the -results of which tests are setout in the graphs forming FIGURES 6 to 8 inclusive, and which referrespectively to Examples 1, II, and HI.

These stress relaxation tests were carried out both with processed wireand also for comparative purposes with unprocessed wire in accordancewith each of these three Examples I, H, and III.

In carrying out the tests on the processed and unprocessed wireaccording to the foregoing three examples, a test length of 29" wastaken in each case and the test specimen was subjected to tension in aknown form of creep testing machine embodying a heating furnacesurrounding the wire so that the test could be carried out at both roomand elevated temperatures, the latter being checked continuously bythree thermocouples spread over the gauge length of the specimen.

in performing the test, the metal of the wire was stressed to a value ofbetween 50% and of the ultimate tensile strength of the wire at roomtemperature, and after loading the Wire to the determined percentagewithin the above range of the ultimate tensile strength, the extensionof the wire under the tensile loading was noted and was thereafter keptconstant by periodically removing a portion of the tensile load appliedto the wire, namely, by removing Weights from the beam of the tensiletesting machine.

The foregoing test was carried out at room temperature, i.e., atordinary atmospheric temperature in the case of each of the threeexamples, l, H, and III, and further tests were performed at an elevatedtemperature of C. in the case of Example I.

Each of the foregoing elevated temperature tests as well as each of theforeoing room temperature tests were performed with the processed andunprocessed wire in each case.

Readings of the applied tensile loading were noted periodically andplotted on a time basis on each of the graphs forming FIGURES 6, 7 and8.

These three graphs show in every case a reduction in the tensile stresswith the length of the wire maintained constant, i.e., the wire wassubjected to the designed conditions of use of a pro-stressing wire, inwhich the wire is embedded in a surrounding matrix of concrete.

Accordingly, the stress relaxation tests .the results of which aredepicted inthe graphs, FIGURES 6, 7 and 8, are a measure of thebehaviour of the wire under service conditions, i.e., as a pro-stressingwire in concrete constructions, and the reduction in the tensile stressboth at room and elevated temperature is a measure of the ability of thewire to maintain its desired pro-stressing characteristics, i.e., itsinitial tensile loading both at room temperature and also when subjectedto moderate heat as may occur during the initial stages of a buildingfire.

Obviously, the usefulness of the wire as a pre-stressing wire .isdenoted by the percentage of the stress reduction or stress relaxationover a given period of time from the initial value; the smaller thereduction the greater the utility of the wire.

In performing the tests carried out at room temperature, both theprocessed and unprocessed lengths of wire, in the case of Example I,having the higher carbon content, namely, .76%, were loaded to aninitial loading of 80 tons per square inch. This initial loading wasfound too great in the case of the tests which were performed withExample I at elevated temperatures, and lower initial loading valueswere employed corresponding to the limit of proportionality in thetensile test, the initial loading selected for each elevated temperaturebeing the same in the case of the unprocessed wire as well as for theprocessed wire at the particular temperature involved.

In the case of the Wire according to Examples II and III having a lowercarbon content, namely, 0.40% and 0.51% respectively, the initialloading of the wire was again of a lesser degree than that employed inExample I at'room temperature, the initial loading being 60 tons persquare inch in the case of Example II and 66 tons per square inch in thecase of Example III.

The stress relaxation for both the processed and unprocessed wire ineach of the three examples at the end of 1000 hours for both roomtemperature and also for the particular elevated temperature tests abovereferred to, is set out in the following Table C, in which table is setout both the initial stress to which the wire was subjected, and alsothefinal stress at the end of 1000 hours while maintaining the length ofwire constant. In the table is also set out the percentage reduction instress necessary to maintain the constant length of the test specimenduring the test, which percentage reduction stress is herein referred toby the term percentage stress relaxation. In this Table C, theabbreviation R.T., is used to indicate room temperature.

Reference to the foregoing Table C and to the three graphs FIGURES 6, 7and 8, clearly show that over the period of each test, namely, 1000hours, in every case the stress relaxation of the processed wire is lessthan that of the unprocessed wire.

Table C Initial Final stress Percentage stress, alter 1,000 stresstons/sq. hours, relaxation inch tons/sq. inch Example I:

Processed R.T 80 78 2. Unprocessed RT 80 74. 3 7. 1 Processed 100 C 6962. 7 9.1 Unprocessed 100 C. 69 55.8 19. 1 Example II:

Processed R.'I 60 58. 3 1 2.8 Unprocessed RJI 60 55 8. 3 Example 111:

Processed R.T 66. 5 65. 7 1.2 Unprocessed R.T 66.5 61. 7 7. 2

1 Less than 3.

From the foregoing table, as well as from the three Further extremelyimportant data which was noted, but which cannot readily be denoted onthe maximum permissible size for the accompanying graphs, is that evenafter 1000 hours, the two curves for the processed and unprocessed wiresof each example at each test temperature, were diverging relatively,that is to say the stress relaxation of the processed wire wascontinuing to decrease at a faster rate than was the case with theunproc essed wire; the increase in stress relaxation which was occurringwith the processed wire at 1000 hours being in every case negligible atroom temperature. Thus as will be seen from the graphs, the curve forthe processed wire in every case is virtually horizontal after theexpiration of 1000 hours, although not quite horizontal in the case ofthe unprocessed wire. i

A further series of tensile tests and stress relaxation or creepresistance tests were carried out on a number of lengths of wireprocessed in accordance with the invention, in which the lengths ofwire, following the cold drawing thereof, were stretched by differingamounts at various temperatures including a number of temperaturesselected within the range of to 500 C.

These wires were all of the same analysis as that of Example I so as tohave a carbon content of 0.76% and a' diameter prior to the processingoperation of 0.201".

The so processed wire was subjected to tensile tests in whichmeasurements were taken of the ultimate tensile stress and the 0.1%proof stress.

The various lengths of wire processed by stretching the lengths bydifferent amounts at varying temperatures, as above referred to, weresubjected to stress relaxation tests in which the lengths of wire werestretched at room temperature, increasing the tensile load by 0.1 tonincrements until the 0.1% proof stress was reached. At this point thetensile stress and strain figures were noted, and it was observed thatthe'wi-re was now creeping, i.e., increasing in length under the appliedtensile stress, and the strain value was now kept constant and over aperiod of 20 hours by decreasing the tensile stress during that pe:riod, a measurement being taken of the tensile stress at the end of the20 hour period with the strain, i.e., the length of the wire, maintainedconstant during this time.

After the expiration of the 20 hour period the samples were subjectedto, maximum tension so as to obtain a measurement of the ultimatetensile stress and also of the ductility as measured by the. percentagereduction in area at the fracture of the specimen.-

The results of'the foregoing series of tests with lengths of wireaccording to Example I, i.e., a carbon content of 0.76% are set out inthe following Table D, the explanation of the abbreviations used inwhich is as follows:

C%tress relaxation test carried out as above described,

followed by tensile test on stress relaxed specimen.

TO'rdinary tensile test on specimen not subjected to stress relaxationtest.

U.T.S.Ultimate tensile stress.

SO0.1% proof stress. a V

SF-Stress in specimen at the expiration of 20 hours after commencementof the stress relaxation test, maintaining the strain of the wireconstant following the attainment of the 0.1% proof stress value.

The ratio of SF to S0 in each of the stress relaxation tests, i.e. creeptests, was evaluated as providing an indication of the reduction intensile stress necessary to maintain the length of the specimenunchanged under the tensile loading.

All of the stress figures given are in tons per square inch.

The whole of the stress relaxation tests, the results of which aresetout in Table D, were carried out at room temperature.

Table D Percent Wire Temp. 01% Stress Nature Stretch, during Proof afterRatio Red. of Tensile i.e. Perma- Stretching, U.T.S. Stress, 20 hrs. SF:50 Area,

Test nent E1on C. SO Creep, Percent gation SF O 1 100 'l 1 100 C- 1 150'l 1 150 C 2 150 T 2 150 C l 200 C 2 200 '1 2 200 C 200 T 5 220 C 1 250T 1 250 C 2 250 T 2 250 C 5 250 T 5 250 C 1 300 T 1 300 C 2 300 T 2 300C 5 300 T 5 300 C 1 400 '1 2 400 C 2 400 T 4 400 C 5 400 C 1 500 C 2 500C 5 500 1 Unprocessed.

An extended stress relaxation test was performed with a length of wireaccording to Example I, i.e., with a carbon content of 0.76%, andstretched at a temperature of 200 C. so as to produce a permanentelongation, i.e., stretch of 2% at that temperature. Two specimens whichwere respectively so processed were taken and initially loaded to atensile stress of 100 and 110 tons per square inch at room temperatureand subjected to stress relaxation tests at this room temperature, whichwas actually C., in a manner similar to the tests already described. Theresult of this further series of tests is set out in the graph formingFIGURE 9.

As will be seen from this figure, the tensile stress in the more highlystressed specimen fell at a more rapid rate during the first few hoursthan the specimen stressed to the lesser initial value and tookconsiderably longer to reach a zero relaxation rate. Nevertheless themore highly stressed specimen attained a zero relaxation ratecorresponding to total cessation of further creep under a stress inexcess of 100 tons at the expiration of less than 8000 hours, while thespecimen stressed initially to the lesser value attained zero relaxationat a tensile load of no less than about 93 tons per square inch afterthe expiration of a little more than 6000 hours.

This result makes plain the great utility of such processed wire as apro-stressing wire in reinforced concrete construction.

It is further plain from each of the foregoing test results that theprocessed wire, i.e. wire which has been subjected to the method thesubject of applicants invention, possesses significantly improved stressrelaxation properties as compared with otherwise identical wire notsubjccted to the method the subject of the invention, so that in the artof pro-stressing wire for concrete, the present invention is ofconsiderable importance.

What we claim then is:

1. A method of increasing the resistance to creep under tension of Wirefor use in reinforced concrete construction, said method comprisingsubjecting plain carbon steel wire having a carbon content within therange of to .9% to a cold drawing operation by advancing the samethrough drawing means, applying tension to the drawn wire to effect itsadvancement through the drawing means,

heating the wire after its emergence from the drawing means and whilesubjected to said drawing tension to a tempering temperature within therange of 220 to 500 C. and maintaining the drawing tension at such valueas having regard to the time during which each wire length is maintainedat the tempering temperature and the value thereof as to impart apermanent elongation to the wire of not more than about 5% and increasethe creep resistance thereof without at the same time applying a tensileforce suificiently great to the wire as to effect failure of the wireunder the applied tensile load.

2. A method of increasing the resistance to creep under tension of plaincarbon steel wire as herein defined for use in reinforced concreteconstruction, said method comprising heating cold drawn plain carbonsteel wire having a carbon content within the range of 0.35% to 0.9% toa temperature within the range of to 400 C., while simultaneouslyapplying a controlled tension to the heated wire, and producing in thewire a permanent elongation of between 1% and 2%.

3. A method of increasing the resistance to creep under tension of plaincarbon steel wire as herein defined for use in reinforced concreteconstruction, said method comprising heating cold drawu plain carbonsteel wire having a carbon content within the range of 0.4% to 0.9% to atemperature within the range of 200 to 400 C., while simultaneouslyapplying a controlled tension to the heated wire so as to produce in thewire a permanent elongation of between 2% and 5%.

4. A method of increasing the resistance to creep under tension of plaincarbon steel wire as herein defined for use in reinforced concreteconstruction, said method comprising subjecting plain carbon steel wirehaving a carbon content within the range of 0.7% to 0.8% to a colddrawing operation by advancing the same through drawing means, applyingtension to the drawn wire to effect its advancement through the drawingmeans and While sub jected to said drawing tension heating said wire toa temperature within the range of 350 to 400 C., and maintaining thedrawing tension at such value as having regard to the time during whicheach wire length is maintained at the drawing temperature and the valuethereof as to impart a permanent elongation to the wire and in 15 creasethe creep resistance thereof without at the same time applying a tensileforce sufficiently great to the wire as to efiect failure of the Wireunder the applied tensile load.

5. A method of increasing the resistance to creep under tension of plaincarbon steel Wire as herein defined for use in reinforced concreteconstruction, said method comprising subjecting plain carbon steel wirehaving a carbon content within the range of 0.4% to 0.8% to a colddrawing operation by advancing the same through drawing means, applyingtension to the drawn wire by means of a power rotated capstan or blockaround which the wire is coiled to effect its advancement through thedrawing means, heating the wire after its emergence from the drawingmeans and while subjected to said drawing tension to a temperaturewithin the range of 250 to 400 C., and maintaining the drawing tensionat such value as having regard to the time during which each wire lengthis maintained at the drawing temperature and the value thereof as toimpart a permanent elongation to the wire and increase the creepresistance thereof without at the same time applying a tensile forcesufficiently great to the wire as to effect failure of the wire underthe applied tensile load, and cooling the wire after it has been heatedto the tempering temperature and before it is coiled around the capstanor block for the purpose specified.

6. A method of increasing the resistance to creep under tension ofcarbon steel wire as herein defined for use in reinforced concreteconstruction, said method comprising subjecting plain carbon steel wirehaving a carbon con-- tent within the range of 0.7% to 0.8% to a colddrawing operation by advancing the. same through drawing means, applyingtension to the drawn wire by means of a power rotated capstan or blockaround which the wire is coiled to effect its advancement through thedrawing means, heating the wire after its emergence from the drawingmeans and while subjected to said drawing tension to a temperingtemperature within the range of 350 to 400 C., and maintaining thedrawing tension at such value as having regard to the time during whicheach wire length is maintained at the drawing temperature and the valuethereof as to impart a permanent elongation to the wire and increase thecreep resistance thereof without at the same time applying a tensileforce sufliciently great to the 'Wire as to efiect failure of the wireunder the applied tensile load, and cooling the wire after it has beenheated to the 1% tempering temperature and before it is coiled aroundthe capstan or block for the purpose specified.

7.'A method of making straightened, stress-relieved, cold drawn steelwire having its'relaxation properties improved which comprisessubjecting cold drawn carbon steel wire having a carbon content of about0.4% to about 0.9% to sufiicient tension to effect a uniform permanentelongation of the wire (of about 1 to 5%) While the wire is heated to atemperature between 150 and 500 C., and cooling the wire to roomtemperature in the straightened condition.

8. A flexible tension member suitable for use in prestressed concretestructures which comprises cold drawn plain carbon steel wire. having acarbon content within the range of 0.4% to 0.9% and having itsrelaxation properties improved by heating the member subsequent to thecold drawing and under sufiicient tension to impart without rupture apermanent elongation of not more than about 5% to the member, to atemperature within the range of 150 to 500 C. such that the member ischaracterized by having a tensile strength of from to more than longtons per square inch, depending upon the carbon content and thetemperature of heating, and a stress relaxation of less than 3% whensubjected at room temperature to aninitial load of at least 70% of itstensile strength and thereafter maintained at a constant length for 1000hours.

References Cited by the Examiner UNITED STATES PATENTS 2,281,132 4/42Young 148-- 12 2,589,881 3/52 Sims et al 14812 2,767,836 10/56 Nachtman148-12 2,816,052 12/57 Hoff et al 14812 OTHER REFERENCES DAVID L. RECK,Primary Examiner.

RAY K. WINDHAM, ROGER L. CAMPBELL,

Examiners.

1. A METHOD OF INCREASING THE RESISTANCE TO CREEP UNDER TENSION OF WIREFOR USE IN REINFORCED CONCRETE CONSTRUCTION, SAID METHOD COMPRISINGSUBJECTING PLAIN CARBON STEEL WIRE HAVING A CARBON CONTENT WITHIN THERANGE OF .35% TO .9% TO A COLD DRAWING OPERATION BY ADVANCING THE SAMETHROUGH DRAWING MEANS, APPLYING TENSION TO THE DRAWN WIRE TO EFFECT ITSADVANCEMENT THROUGH THE DRAWING MEANS, HEATING THE WIRE AFTER ITSEMERGENCE FROM THE DRAWING MEANS AND WHILE SUBJECTED TO SAID DRAWINGTENSION TO A TEMPERING TEMPERATURE WITHIN THE RANGE OF 220* TO 500* C.AND MAINTAINING THE DRAWING TENSION AT SUCH VALUE AS HAVING REGARD TOTHE TIME DURING WHICH EACH WIRE LENGTH IS MAINTAINED AT THE TEMPERINGTEMPERATURE AND THE VALUE THEREOF AS TO IMPART A PERMANENT ELONGATION TOTHE WIRE OF NOT MORE THAN ABOUT 5% AND INCREASE THE CREEP RESISTANCETHEREOF WITHOUT AT THE SAME TIME APPLYING A TENSILE FORCE SUFFICIENTLYGREAT TO THE WIRE AS TO EFFECT FAILURE OF THE WIRE UNDER THE APPLIEDTENSILE LOAD.